Laser beams are applied to various fields today. For example, a laser processing technique is developing rapidly. The laser processing technique is desired to provide processing to a smaller material with a higher precision. It is necessary to condense a laser beam into a smaller spot. To attain this, a method is conceivable in which a numerical aperture of a condensing optical system is increased, or an F number of the condensing optical system is decreased to provide a lens with a high brightness. When a lens with a small focal length and a high brightness is used, a spot size is decreased in proportion to the focal length as long as aberration is suppressed; however, a focal depth is decreased. Hence, an extremely high precision is required during focusing onto a surface of a workpiece. In addition, it is difficult to process a material with a large thickness. To address such a problem, a condensing optical system is suggested, in which a focal depth is increased without a spot size being changed.
For example, Japanese Patent No. 2664625 discloses a condensing optical system in which a lens surface is concentrically divided into a plurality of subfields, so that the lens serves as a multifocal lens having the subfields with slightly different focal lengths. Positions of light spots condensed by the subfields are mutually shifted from each other, and a focal depth is apparently increased. Also, for example, Japanese Unexamined Patent Application Publication No. 9-64444 discloses a condensing optical system which employs a Bessel beam. This beam is condensed into a very small spot, and a focal depth is markedly increased.
However, with the condensing optical system disclosed in Japanese Patent No. 2664625, the laser beam is divided at the lens surface and their focuses are discontinuously connected. The spot sizes and intensity distributions may vary in the subfields. Hence, an intensity distribution may be discontinued in front and rear areas of a focal position. With the condensing optical system disclosed in Japanese Unexamined Patent Application Publication No. 9-64444, although the Bessel beam has the large focal depth, a laser beam condensed into a certain spot is merely a part of entire incident light. Thus, the intensity of the spot is considerably small, and is not suitable for processing which requires high intensity.
Meanwhile, a brittle material, such as single crystal diamond, sintered polycrystalline diamond, sintered cubic boron nitride, or cemented carbide, includes a highly covalent substance and has a high melting point and a high hardness. Hence, it is difficult to machine such a material. Owing to this, the above-described condensing optical system is used to cut the brittle material or to form a groove in the brittle material by irradiating the brittle material with a laser beam (for example, see Japanese Patent No. 3449986, Japanese Unexamined Patent Application Publication No. 2003-62683, or Japanese Patent No. 3616872).
For example, when a brittle material is to be cut, a condensing lens condenses a laser beam emitted from a laser oscillator, and the brittle material arranged at a focal position of the condensing lens is irradiated with the condensed laser beam, to cut the brittle material.
Regarding the condensing optical system to obtain the laser beam, a conventional condensing lens formed of a spherical lens has a characteristic that, when a laser beam whose light intensity distribution is in an ideal Gaussian form is incident, the form of the laser beam is converted into a reduced Gaussian form as shown in FIG. 40.
When a brittle material is cut with a laser beam passing through the condensing lens with the characteristic, a defect, such as chipping, taper (inclination with respect to an expected work surface), or dulling, may occur in a cut surface at a laser exit side. This is possibly because the laser beam exhibits change in beam diameter or change in optical power density, the change being symmetric in an optical-axis direction about a light condensed point (see FIG. 41). When a brittle material is laser processed to make a cutting tool, merely cutting the material is insufficient. Finishing precision of an “edge” is essential to a cutting tool. If a defect occurs at an edge portion, repairing the defect may need considerable work hours and costs.
Thus, in cutting using the conventional condensing lens, in order to suppress occurrence of chipping or the like, the number of laser scanning operations is increased, the width of cutting is increased, or both measures are employed to allow the laser beam to sufficiently reach the inside of a brittle material.
However, when the number of laser scanning operations is increased, a processing speed is decreased. When the width of cutting is increased, a loss of the material is increased. Further, with the conventional method, although the occurrence of taper or dulling may be suppressed by a certain degree, it is difficult to completely prevent such a defect.